The present invention relates to a process for converting hydrocarbons, for example in refining or petrochemistry, or the fine chemicals industry, in the presence of a catalyst, carried out in a three-phase reactor and in which the liquid Peclet number (Pe1) is in the range 0 (excluded) to about 10.
More particularly, the present invention relates to chemical conversion processes with relatively high exothermicity or endothermicity, which thus require thermal reaction control.
More particularly, the present invention is applicable to exothermic reactions producing a high temperature, for example above 100xc2x0C. and usually above 130xc2x0C., generally at an absolute pressure of more than 0.1 megapascals (MPa) and usually more than 0.5 MPa.
Reactions with marked exothermicity or endothermicity that can be cited include dehydrogenation, hydrogenation, transhydrogenation, the decomposition of ethers to olefins, the hydrogenolysis of hydrocarbons or esters, aromatisation, hydrodenitrogenation, hydrotreatment, in particular residue hydrotreatment, hydroformylation, alcohol synthesis, polyolefin synthesis, isomerisation, oligomerisation, in particular dimerisation (such as the Applicant""s dimersol processes), oxidation, hydrodesulphurisation and aliphatic or aromatic alkylation. Some of these reactions have been described, for example, in J. F. LePage""s book published by Technip in 1987 entitled xe2x80x9cApplied Heterogeneous Catalysisxe2x80x9d and in xe2x80x9cUllmann""s Encyclopaedia of Industrial Chemistryxe2x80x9d (volume B4, 5th Edition, pages 105-106) cited by way of reference, the descriptions of which are hereby considered to be included in the present description by dint of this reference which are hereby considered to be included in the present description by dint of this reference thereto.
Several types of chemical conversion reactors can be used, the solid being used either in an entrained bed or in a bubble column reactor or slurry bubble column where gas comes into contact with a liquid/very finely divided solid mixture (slurry). The term xe2x80x9cslurryxe2x80x9d will be used in the present description to designate a suspension of solid particles in a liquid. The heat of reaction is normally eliminated by a cooling exchanger generally inside the reactor.
Within the context of the present invention, the reactor used is preferably a slurry bubble column. Slurry bubble columns comprise a liquid medium containing solid particles in suspension, generally mainly catalytic particles, comprising at least one means for introducing at least one gas phase comprising at least one reactant, by means of at least one distribution means that produces gas bubbles normally with a relatively small diameter. The gas bubbles rise in the column and at least one reactant is absorbed by the liquid and diffuses towards the solid. When the solid is a catalyst, the reactants are converted on contact into gaseous and/or liquid and/or solid products depending on the conversion conditions and catalyst type.
The gaseous products which may comprise at least one unconverted gaseous product and gaseous products that may be formed during the reaction, are collected near the head of the column. The suspension, containing the liquid acting to form the suspension of the solid and the liquid products formed during the conversion, is recovered via a line generally close to the upper level of the suspension in the column.
The solid particles are then separated from the liquid using any means known to the skilled person, for example by filtering. A description of the operation of a slurry bubble column can be found in the Applicant""s European patent application EP-A-0 820 806 cited by way of reference, the description of which should hereby be considered to be included by dint of this reference.
Facilities for carrying out the process of the invention also generally comprise separation means for the production of a liquid fraction and possibly also of residual gaseous products or of products formed during conversion, and possibly inert compounds.
The catalysts used in these processes and methods for producing such catalysts are well known to the skilled person. A variety of such catalysts can be used, and usually contain at least one metal selected from metals from groups 5 to 11 of the new periodic table (groups Ib, Vb, VIb, VIIb and VIII of the old periodic table).
The support is generally a porous material and usually a porous inorganic refractory oxide. By way of example, the support can be selected from the group formed by alumina, silica, titanium oxide, zirconia, rare earths or mixtures of at least two of these porous minerals.
The desired products are generally separated substantially completely from the catalyst (for example until the amount of residual catalyst is of the order of 1 to a few parts per million (ppm)), to enable its use or treatment in subsequent steps.
Typically, the quantity of solid particles of catalyst in the slurry represents 10% to 65% by weight of the slurry. These particles usually have a mean diameter in the range about 10 to about 800 microns. Finer particles may be produced by attrition, i.e., fragmentation of the initial catalyst particles.
European patent application EP-A-0 450 861 describes the use of a catalyst based on cobalt dispersed on titanium oxide in a slurry bubble column type reactor. Further, EP-B-0 450 860 describes a method for operating that type of reactor in an optimal manner.
Those two documents indicate that the performance of the catalysts essentially depends on the concentration of gaseous reactant (synthesis gas) in the reactor, i.e., on the partial pressure of carbon monoxide and hydrogen in the reaction zone.
In hydrodynamics terms, those documents then indicate that in a perfectly mixed reactor, such as a fully back-mixed reactor or CSTR, the composition of gaseous reactants and liquid and gaseous products and the concentration of catalyst are the same at any point in the reactor. Thus, those perfectly mixed reactors lead to better selectivity for C5+hydrocarbons, but to the detriment of productivity.
In contrast, in a plug flow reactor, the partial concentration of reactant decreases along the entire length of the reaction zone, and that type of reactor results in the highest productivities to the detriment of selectivity.
EP-B-0 450 860 indicates that Peclet numbers for the gas phase of more than 10, also known as xe2x80x9cgas Peclet numbers or Pegxe2x80x9d, lead to a plug flow type operation regarding the gas phase, while gas Peclet numbers (Peg) of less than 1 correspond to systems in which the gas phase is perfectly mixed or stirred. Ideal perfectly stirred systems correspond to Peclet numbers tending towards zero. This Peclet number is equal to Peg=H ug/Dax, where H is the expansion height of the catalytic bed, ug is the space velocity of the gas and Dax is the axial dispersion coefficient of the gas phase.
The method that can produce an optimal slurry bubble column that is described in EP-B-0 450 860 comprises injecting gas at a mean superficial velocity such that the formation of slug flow is avoided, the gas superficial velocity being 0.2 (H/Dax) or more. A further condition applies to the superficial velocity of the liquid and the sedimentation rate of the solid (generally the catalyst) so that the solid is suitably fluidised in the liquid phase.
Those documents do not take thermal effects into account, nor the presence of an undesirable reaction that has a large negative influence on the exothermicity and selectivity of the reaction. Too much exothermicity in the catalyst generally leads to an increase in the formation of methane, a product that is favoured by high temperatures, and a drop in activity, for example by sintering of the active phase (M. E. DRY, xe2x80x9cCatalysis Science and Technologyxe2x80x9d, Volume 1, Anderson and Boudart, pages 175 and 198).
The invention concerns a process for converting hydrocarbons by reaction in the presence of a catalyst, carried out in a three-phase reactor and in which the liquid Peclet number (Pe1) is in the range 0 (excluded) to about 10, preferably in the range from about 0.005 to about 8, more preferably in the range from about 0.01 to about 5 and highly preferably in the range from about 0.02 to about 3 or in the range from about 0.03 to about 1.
This process can control the reaction on a thermal level and encourage formation of the desired products.
The invention concerns a process for coverting hydrocarbons in a reaction zone containing a suspension of solid particles in a liquid, which comprises solid particles of catalyst for the reaction. Said suspension is also termed a slurry. The process of the invention is thus carried out in a three-phase reaction. Preferably, the process of the invention is carried out in a slurry bubble column type three-phase reactor.
The Applicant has discovered that it is important to be able to control the hydrodynamics of the liquid if thermal transfers are to be controlled in the reaction zone, as well as the reaction itself.
In the process of the invention, it is the reactant, liquid or dissolved in the liquid phase, that comes into contact with the catalyst in suspension in said phase and which reacts.
Regarding mass transfer it is preferable to establish a flow rate in the reactive phase, and thus the liquid phase that contains dissolved gas, that is as close as possible to plug flow in order to obtain maximum conversion. However, in the case of highly exothermic or highly endothermic reactions, plug flow generates a substantial temperature profile that renders thermal control difficult.
The mixture of reactants entering the reactor undergoes the reaction and this continues as the fluid advances into the column. In this type of reactor function, the concentration and partial pressure of the reactants reduces along the reactor while that of the products (gaseous or liquid) and the water produced by the reaction increases. Plug flow is thus the origin of a concentration gradient associated, in the case of a highly exothermic or highly endothermic reaction, with a substantial temperature gradient along the reactor.
Consider the following reaction scheme, which is well known to the skilled person, for an endothermic reaction: 
When the parallel reaction, formation of undesirable products C (reaction 2), has an activation energy (E2) which is higher than that (E1) of the main reaction of formation of desirable products B, the rate of formation of methane increases faster with temperature than that of other hydrocarbons. Further, since the two reactions are exothermic (enthalpies xcex94H1 and xcex94H2 for reactions 1 and 2 are negative), progress of the reaction causes an increase in the heat released by the reaction, which increases the temperature and thus the formation of undesirable products C.
An increase in the temperature gradient along the reactor thus results in a reduction in the selectivity for desired products. A similar reasoning can be made in the case of endothermic reactions where E1 greater than E2 (E1 is the energy of activation of the reaction leading to undesirable products, and E2 is the energy of activation of the reaction leading to undesirable products C). a reduction in the temperature due to the endothermicity of the reaction encourages formation of products C.
The present invention describes a process whereby the formation of desirable products B by reaction 1 is encouraged by controlling the parameters associated with the reaction.
In the case of the reaction scheme described above (reactions 1 and 2), the unsteady state material balance equations can be written as:
Species A:                     1                  Pe          l                    ·                                    ∂            2                    ⁢                      C            A                                    ∂                      Z            2                                -                  ∂                  C          A                            ∂        Z              -          (                                    r            1                    ⁡                      (                          C              A                        )                          +                              r            2                    ⁡                      (                          C              A                        )                              )        =            ∂              C        A                    ∂              t        *            
Species B:                     1                  Pe          l                    ·                                    ∂            2                    ⁢                      C            B                                    ∂                      Z            2                                -                  ∂                  C          B                            ∂        Z              +                  r        1            ⁡              (                  C          A                )              =            ∂              C        B                    ∂              t        *            
Species C:                     1                  Pe          l                    ·                                    ∂            2                    ⁢                      C            C                                    ∂                      Z            2                                -                  ∂                  C          C                            ∂        Z              +                  r        2            ⁡              (                  C          A                )              =            ∂              C        C                    ∂              t        *            
Similarly, the unsteady state energy balance can be written as:                     1                  Pe          T                    ·                                    ∂            2                    ⁢          T                          ∂                      Z            2                                -                  ∂        T                    ∂        Z              +                  (                                                            r                1                            ⁢                              (                                  C                  A                                )                                      ·                          (                                                -                  Δ                                ⁢                                  xe2x80x83                                ⁢                                  H                  1                                            )                                +                                                    r                2                            ⁢                              (                                  C                  A                                )                                      ·                          (                                                -                  Δ                                ⁢                                  xe2x80x83                                ⁢                                  H                  2                                            )                                      )            ·              τ                  ρ          ·                      C            p                                -                            U          ·          a          ·          τ                          ρ          ·                      C            p                              ·              (                  T          -                      T            cool                          )              =            ∂      T              ∂              t        *            
where                     Pe        l            =                                                                  u                l                            ·              H                                      D              ax                                ⁢                      xe2x80x83                    ⁢                      Pe            T                          =                                            ρ              ·                              C                P                                      λ                    ·                      u            l                    ·          H                      ⁢          xe2x80x83            Z    =                            z          H                ⁢                  xe2x80x83                ⁢        τ            =                                    H                          u              l                                ⁢                      xe2x80x83                    ⁢                      t            *                          =                                            t              τ                        ⁢                          xe2x80x83                        ⁢                                          r                i                            ⁡                              (                                  C                  A                                )                                              =                                    k              o                        ·                          e                                                E                  i                                                  R                  ·                  T                                                      ·                          C              A              n                                          
Given that the thermal dispersion very closely follows the mass dispersion, equality of the thermal Peclet number and the mass Peclet number for the liquid phase Pe1 constitutes a reasonable hypothesis that is accepted by the skilled person (P. L. MILLS et al., xe2x80x9cThree-Phase Sparged Reactorsxe2x80x9d in Topics in Chemical Engineering, volume 8, chapter 5, p. 364, K. D. P. NIGAM and A. SCHUMPE editors, GORDON and BREACH, publishers). The selectivity for product B and the temperature profile along the column can then be determined by solving the above equations.